A urethane (meth)acrylate has hitherto been produced by a batch process using a reaction vessel. In general, a urethane (meth)acrylate is produced by reacting a hydroxyl group of a compound having a hydroxyl group and a (meth) acryloyl group with an isocyanate group of a compound having an isocyanate group. The reaction between a hydroxyl group and an isocyanate group is exothermic reaction. In the process of the exothermic reaction proceeding, when the temperature of the reaction system is abnormally increased due to accumulation of the reaction heat, the (meth)acryloyl groups are bonded to each other to cause runaway reaction where the reaction product is gelled. Accordingly, for suppressing excessive generation and accumulation of the reaction heat, which bring about runaway reaction, precise temperature control and efficient removal of heat are demanded, but in the case of a large reaction system, such as an industrial production scale, it is difficult to achieve them due to efficiency of agitation, heat removal capability and the like.
The runaway reaction occurring brings about gasification of the unreacted raw materials due to heat generation on the runaway reaction, and gasification due to decomposition of the urethane (meth)acrylate produced. The phenomena may result in quick increase of the pressure inside the reaction system to cause fatal breakage of the equipment in some cases. For preventing the equipment from being broken, a measure of opening the pressure accumulated in the system and a measure of discharging the contents to the outside of the system are taken by opening a manhole, a safety valve, a rupture disk, an explosion vent or the like provided on the reaction vessel. However, the measures require large-scaled equipments, which bring about an increased investment cost and a large occupied area, and thus only poor productivity is obtained.
A method of adding a polymerization inhibitor is known as a measure for terminating runaway reaction occurring during production of a urethane (meth)acrylate. It is necessary in the method that a polymerization inhibitor is dispersed and dissolved in the reaction system in a very short period of time.
However, the reaction system is liable to be increased in viscosity upon gelling, which bring about such a problem that it is considerably difficult to disperse and dissolve a polymerization inhibitor in the reaction system in a very short period of time. In addition, the driving capacity of the agitation blade for agitating the reaction system becomes insufficient against the quick increase of the viscosity due to the runaway reaction, which results in not only the failure of sufficient dispersion and dissolution of the polymerization inhibitor in the reaction system, but also such a problem that the equipment suffer fatal breakage, such as malfunction of the agitating device due to application of load. The problem becomes serious in the case where a diluent solvent or the like is not used upon producing a urethane (meth)acrylate.
As a measure for facilitating dispersion and dissolution of a polymerization inhibitor in a reaction system, such a method may be used that a polymerization inhibitor is used after dissolving in a solvent or the like, but for addressing occurrence of runaway reaction, it is necessary to prepare in advance a large amount of a solution containing a polymerization inhibitor dissolved in a solvent. Accordingly, the cost of production of a urethane (meth)acrylate is increased. A polymerization inhibitor contains a mutagenic substance and thus is necessarily handled with adequate care.
As a measure for suppressing runaway reaction from occurring upon production of a urethane (meth)acrylate, such a method has been known that a solvent is made present in the reaction system, thereby decreasing the concentration of the polymerizable compound in the system and simultaneously decreasing the viscosity of the fluid, which is increased associated with the progress of the reaction, for enhancing the controllability of the reaction. In the method, however, the solvent is necessarily recovered after completing the reaction, and the yield ratio per reaction vessel is decreased due to the use of the solvent, which impair the productivity. Furthermore, the resin composition thus obtained may have a problem in quality due to the solvent remaining in a slight amount therein.
The production of a urethane (meth)acrylate may not have good productivity due to the aforementioned measures for addressing runaway reaction. Furthermore, even with the measures for addressing runaway reaction performed, it is the current situation that a urethane (meth)acrylate is produced with poor efficiency under conditions that do not cause runaway reaction, for example, at a low temperature of 100° C. or less, preferably 80° C. or less, for a prolonged period of time of from 5 to 7 hours (see, for example, Patent Document 1).
As a production method capable of preventing heating with the reaction heat, suppressing side reaction, such as multimerization, and providing a high reaction rate, such a method for producing a ring-opening reaction product of an epoxy compound is proposed that an epoxy compound and a compound having a hydroxyl group are caused to pass through a channel having a fluid cross-sectional area of from 300 to 1,000,000 μm2 (from 3×104 to 1 mm2), and the compounds are made in contact with each other to perform ring-opening addition reaction (see, for example, Patent Document 2). However, the production method disclosed in Patent Document 2 is limited to the ring-opening reaction of an epoxy compound, and fails to disclose any measure for preventing runaway reaction in polymerization of a (meth)acryloyl group, which is an issue in the production of a urethane (meth)acrylate Furthermore, a long period of 160 minutes is required for reacting the epoxy compound and the compound having a hydroxyl group sufficiently (for example, to a reaction rate of 98%) in the channel, and thus the production efficiency is considerably poor.    Patent Document 1: JP-A-08-109230    Patent Document 2: JP-A-2006-111574